[2R-[2R*[R*(R*)]]]-α-[(aminophenylacetyl)amino]-5-chloro-3,4-dihydro-4-oxo-2H-1,3-Thiazine-2-acetic Acid is an impurity of cefaclor (C235250), which is a second-generation cephalosporin antibiotic. Antibacterial.